The present invention relates to a method for operating an internal combustion engine.
By way of example, DE 195 19 663 A1 has disclosed a method for operating an internal combustion engine with spontaneous ignition, in which in a first stage a homogenous, precompressed fuel/air mix which is not suitable for spontaneous ignition is provided in the working space, and in a second stage an additional quantity of the same fuel is injected into the working space in order to bring about the spontaneous ignition. In this case, the fuel/air mix is prepared by means of external mix formation and introduced into the working space, where it is compressed to close to the spontaneous ignition point. The injection of the additional quantity of fuel in the second stage takes place in finely atomized form, avoiding contact with the walls, so as to form a mix cloud in which, on the one hand, the fuel/air ratio is no greater than the stoichiometric mixing ratio and in which, on the other hand, the spontaneous ignition condition is achieved.
Furthermore, DE 198 52 552 C2 has disclosed a method for operating a four-stroke internal combustion engine which, at part-load, forms a lean base mix from air, fuel and retained exhaust gas and, at full load, forms a stoichiometric mix. At part load, compression ignition takes place, whereas at full load spark ignition takes place. Furthermore, mechanically controlled exhaust-gas retention with switchable valve closure overlap and exhaust gas throttling is provided. An activation injection can be carried out into the retained exhaust gas. The quantity of retained exhaust gas, with the valve closure overlap switched on, is controlled or preset as a function of the engine speed and engine load by an exhaust-gas throttle valve which is active for all the combustion chambers. The pressure when the intake members open into the individual combustion chambers is evened out by a cylinder-selective, cylinder-consistent activation injection.
A method for operating a four-stroke, reciprocating-piston internal combustion engine is also known from DE 198 18 569 C2. It is characterized by a homogenous, lean base mix of air, fuel and retained exhaust gas and by compression ignition and direct injection of the fuel into the combustion chamber. The volume of the combustion chamber changes cyclically. The combustion chamber can be filled with fresh gas through at least one intake member, while the combustion exhaust gases can be at least partially expelled through at least one exhaust member. In the part-load range and in the lower full-load range, the internal combustion engine is operated with compression ignition and preferably mechanically controlled exhaust-gas retention, whereas in the full-load range and high part-load range it is operated on the Otto cycle.
One drawback of the methods known from the abovementioned documents is in particular that during the compression ignition of homogenous mixes, the start of combustion and the release of heat cannot be controlled. The control of the exhaust-gas quantity by exhaust-gas retention and exhaust-gas suck-back are expensive and difficult to realize.
By contrast, an object of the present invention is to provide a method for operating an internal combustion engine in which the reaction state of the mix can be influenced in a targeted way.
This object has been achieved by providing the mass of fuel to be injected is divided into a preinjection and a main injection, the preinjection taking place into the intermediate compression stroke and the main injection taking place synchronously with the induction. This type of division of the injected fuel masses allows the combustion position to be influenced in a targeted way, because this preinjection into the intermediate compression stroke can be used to influence the temperature and composition of the working gas and therefore the reaction state of the mix.
In a further configuration of the invention, the mass ratio of the fuel mass injected in the preinjection and the fuel mass injected in the main injection is divided according to the operating state of the internal combustion engine. In a currently preferred embodiment of the invention, the division of the injected fuel masses into preinjection and main injection is approximately 50:50.
In a further configuration of the invention, the valve closure overlap between the intake and exhaust valves remains constant during load changes.
Furthermore, it is advantageous if the injection point of the preinjection is dependent on the rotor speed and the injection pressure.